1. Field of the Invention
The present invention relates to a bi-section semiconductor laser device, a method for manufacturing the same, and a method for driving the same.
2. Description of the Related Art
It is expected that a high-power ultrashort-pulse semiconductor laser device that has an emission wavelength of 405 nm and is composed of a GaN compound semiconductor will realize a light source of a volumetric optical disk system expected to be a next-generation optical disk system of the Blu-ray optical disk system or a light source demanded in medical and bio-imaging fields. There are mainly two methods for generating short-pulsed light in a semiconductor laser device, namely, gain switching and mode locking. Mode locking is further classified into active mode locking and passive mode locking. To generate an optical pulse using active mode locking, an external cavity has to be obtained using a mirror and a lens and radio-frequency (RF) modulation has to be applied to the semiconductor laser device. On the other hand, in passive mode locking, an optical pulse can be generated through simple direct current driving by using self-pulsation (self-oscillation) of a semiconductor laser device.
To operate a semiconductor laser device through self-pulsation, a light-emitting region and a saturable absorption region have to be formed in the semiconductor laser device. Herein, in accordance with the arrangement of the light-emitting region and the saturable absorption region, a semiconductor laser device can be classified into a saturable absorber layer (SAL) type or a weakly index guide (WI) type in which the light-emitting region and the saturable absorption region are arranged in a vertical direction or a bi-section type in which the light-emitting region and the saturable absorption region are arranged side by side in a cavity direction. A bi-section semiconductor laser device is common as disclosed in Japanese Unexamined Patent Application Publication Nos. 2004-007002 (Patent Document 1), 2004-188678 (Patent Document 2), and 2008-047692 (Patent Document 3). A bi-section GaN semiconductor laser device has a large saturable absorption effect and can generate a narrow optical pulse compared with a SAL semiconductor laser device.
In general, a bi-section GaN semiconductor laser device includes:
(a) a stacked structure obtained by stacking, in sequence, a first compound semiconductor layer of a first conductivity type composed of a GaN compound semiconductor, a compound semiconductor layer that constitutes a light-emitting region and a saturable absorption region each composed of a GaN compound semiconductor, and a second compound semiconductor layer of a second conductivity type different from the first conductivity type composed of a GaN compound semiconductor;
(b) a belt-shaped second electrode formed on the second compound semiconductor layer; and
(c) a first electrode electrically connected to the first compound semiconductor layer.
The second electrode is separated by a separating groove into a first portion configured to provide a forward bias state by supplying a DC current to the first electrode through the light-emitting region and a second portion configured to apply an electric field from the first electrode through the saturable absorption region.